The invention relates to an evaporator, a method of evaporating an evaporable liquid, and individual evaporator effects, that are particularly advantageous for a number of particular processes. The invention is particularly applicable to--although not limited to--desalination of water, wash water, and evaporation of black liquor during paper pulp processing.
Conventional evaporators for water desalinization typically comprise multiple effect evaporators having a plurality of heat transfer tubes. Both vertical tube evaporators and horizontal tube evaporators are utilized. The brine is typically sprayed on the outsides of the tubes to produce vapor. The water vapor generated by the brine flows to the next effect, where it supplies heat for additional evaporation at a lower temperature. The brine may be partially preheated by a fraction of the vapor, and flow countercurrently with the steam. Most often prime steam and downstream vapors flow inside the horizontal tubes, where they condense and contribute to the product water stream.
Conventional multi-effect evaporators used for evaporating black liquor from paper pulp processing are the vertical type, having vertically elongated dimpled plates as the heating elements, such as plates shown--for example--in U.S. Pat. Nos. 3,211,219 and 3,512,239 (the disclosures of each are incorporated by reference herein).
According to the present invention there is provided a multi-effect evaporator, a method of effecting evaporation utilizing a multiple effect evaporator, and individual effects of an evaporator, which have a number of advantageous features. Each of the effects according to the present invention comprises horizontally elongated dimpled plates. The dimpled plates are disposed in one or more sets, one stacked above the other. The dimpled plate horizontal element construction has a uniform liquor distribution, relatively little vapor pressure drop between the heating elements, and good droplet separation (minimized carryover), as well as high heat transfer coefficients. Thus it is advantageous compared to conventional horizontal tubular evaporators. Also, it has an economic advantage with respect to the known vertical dimpled plate evaporators used in paper pulp processing. According to the invention 15 to 20% better packing can be achieved in the same space than for comparable vertical dimpled plate evaporators, resulting in a generally corresponding savings in capital investment.
According to one aspect of the present invention there is provided a method of evaporating an evaporable liquid utilizing a multiple effect evaporator. In the multiple effect evaporator each effect has one or more (e.g. first and second) sets of horizontally elongated dimpled plate evaporator elements, the elements as shown in said U.S. Pat. Nos. 3,211,219 and/or 3,512,239. Each set has an internal passage and an external surface, and the first set is disposed on top of the second set, etc. The effects are connected in series and include a first, highest temperature effect, and a last, lowest temperature effect. The method comprises the following steps (when using first and second sets): (a) Introducing horizontally flowing heating fluid into the internal passage of both sets of elements of the first effect, and into the internal passage of at least the second set in all subsequent effects. (b) Providing a head of circulation liquid to be evaporated at the top of the first set of each effect. (c) Causing the liquid to flow downwardly from the head over the external surfaces of the first and second sets of each effect. (d) Withdrawing the vapor of evaporated circulation liquid from each effect while demisting it to separate entrained liquid from the vapor. (e) Recirculating circulation liquid that has not evaporated to the head associated with that effect. (f) Horizontally feeding the withdrawn vapor from each effect to the next effect as the fluid provided in step (a) thereof, except for the last effect. And, (g) withdrawing clean condensate from the last effect. There may also be the further step (h) of preheating the liquid to be evaporated before introducing it into the head of the first effect by passing it through the internal passages of the first sets of all the other effects except the first effect, the liquid flowing countercurrent to the vapor passing from the first effect to the last effect through the second sets' internal passages. There may also be the further step (i) of compressing the vapor withdrawn from the last effect to increase the temperature thereof, and providing the compressed vapor to the first effect as the heating fluid in step (a). Still further there may be the further step (j) of removing non-condensable gases from each effect, and recovering a major portion of the heat value from the non-condensable gases before discharge thereof. For each effect except the last effect, between steps (f) and (a) there may be an additional step of collecting condensate, and passing and flashing the condensate so collected to the next effect.
The invention is particularly useful in the desalinization of water, to produce fresh water from sea water, but may also be used to evaporate black liquor from a paper pulp producing procedure, or act upon like liquors.
According to another aspect of the present invention an evaporator effect is provided. The evaporator effect comprises: A horizontally elongated vessel, having a top, bottom, first end, and second end. One or more sets of elongated (in a dimension of elongation) dimpled plate evaporator elements, defining an interior passage, and having an exterior surface. When it is provided, a second set of elongated in a dimension of elongation dimpled plate evaporator elements also defines an interior passage, and has an exterior surface. Means for introducing heating fluid into the interior passage of at least the second set of dimpled plate evaporator elements. Means for mounting the first and second sets of dimpled plate evaporator elements so that they are disposed in the vessel with their dimension of elongation horizontal, with the first set above the second set. Means for introducing circulation liquid to be evaporated adjacent the top of the vessel so that it falls over the exterior surfaces of the first and second sets of elements in a falling film, to cause some of the circulation liquid to be evaporated. A demistor mounted within the vessel. Means for withdrawing evaporated circulation liquid from the vessel through the demistor. Means for causing circulation liquid not evaporated to collect separately at the bottom of the vessel. And, a circulation liquid outlet from the bottom of the vessel. The means for introducing circulation liquid preferably comprises an open top, closed bottom tray mounted above the elements, with means defining a plurality of openings in the tray bottom. Means also may be provided for introducing liquid to be preheated into the interior passage of dedicated (e.g. the first set of) elements, to flow countercurrently to the fluid introduced into the interior passage of the second set of elements.
According to still another aspect of the present invention, a multi-stage evaporator having a plurality of effects which are interconnected into a series of effects is provided. The series of effects includes a first, highest temperature, effect to a last, lowest temperature, effect. Each effect in the series is as described above. For each effect, except the first effect, the means for introducing heating fluid is connected to the means for withdrawing evaporated circulation liquid from the previous effect. A dividing wall is also provided adjacent the elements of each effect to define--with the bottom of the vessel--a condensation chamber. An outlet is provided from the bottom of the condensation for passing this "clean" condensate to the similar condensate chamber to flash in the next effect, and exit from the last effect.
It is the primary object of the present invention to provide for effective evaporation of an evaporable liquid utilizing a multiple effect evaporator. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.